1. Field of the Invention
The present invention relates to an ion shower apparatus for use in the process of etching fine patterns in the manufacture of semiconductor devices, for use in the process of sputtering a target by an ion current, for use in the process of ion beam deposition on a specimen surface of the like.
2. Description of the Prior Art
The ion shower apparatus of the type described above comprises, in general, a plasma formation chamber for producing a plasma producing ions; ion extraction grids so disposed as to partially define the plasma formation chamber thereby extracting the ions from the plasma formation chamber so as to produce an ion beam in the form of a shower (to be referred to as "a shower-like ion beam" in this specification) and a specimen chamber in which the surface of a specimen is irradiated with the shower-like ion beam. Such an ion shower apparatus is widely used in the manufacture of semiconductor devices by using an ion etching or reactive ion etching method, an ion beam sputtering method or an ion beam deposition method.
FIG. 1 shows a Kaufman type ion shower apparatus which has been widely used. Such an apparatus is disclosed in detail, for example, in "Ion Milling for Semiconductor Production Processes", L. D. Bollinger; Solid State Technology, Nov., pp. 66-70 (1977). In FIG. 1, reference numeral 1 designates a plasma formation chamber; 2, a specimen chamber; 3, a thermionic cathode; 4, an anode; 5, an etching gas inlet; 6 and 7, an upper and a lower ion extraction electrode or grid; 8, a specimen or substrate table; 9, an object to be subjected to etching, deposition or sputterring, for example, a specimen or substrate; 10, plasma; 11, a shower-like ion beam; 12, a solenoid magnet; and 13, a mask.
In operation, an etching gas is introduced through the inlet 6 into the plasma formation chamber 1 and is ionized by electrones accelerated from the thermionic cathode 3 to the anode 4. The solenoid magnet 12 is used to increase an ionization efficiency.
Each of the ion extraction electrodes or grids 6 and 7 has a plurality of apertures having a diameter of 2-3 mm and a high potential (for instance, 1000 V) is applied to the plasma formation chamber 1 and to the upper electrode 6, while the lower electrode 7 is grounded. The boundary of the plasma sheath is defined in the vicinity of the upper ion extraction grid 6 by the grid 6. The ionized etching gas in the plasma 10 flows into the sheath boundary and is then accelerated by the electric field in the plasma sheath, so that the ionized etching gas is extracted to the specimen chamber 2. As a result, in case that the shower-like ion beam 11 is used for the purpose of etching, the shower-like ion beam 11 impinges against the specimen or substrate 9 on the table 8, so that the surface of the specimen or substrate 9 which is not covered by the mask 13 is physically or physio-chemically etched. Further, the mask 13 is to be selected from materials which do not react with the shower-like ion beam 11.
The dimensions of the two ion extraction grids 6 and 7 and the ion beam convergence at these grids are discussed in detail in "Ion Beam Divergence Characteristics of Two-Grid Accelerator Systems", by G. Aston et al., AIAA JOURNAL, Vol. 16, No. 5, pp. 516-524.
FIG. 2 shows an ion source of a prior art ion shower apparatus of the type in which plasma is produced by electron cyclotron resonance excited by microwave. The construction of the apparatus is disclosed in detail in Japanese Patent Application No. 61,409/1981. In FIG. 2, reference numeral 21 denotes a plasma formation chamber; 22, a specimen chamber; 23, an ion extraction grid assembly having an upper floating electrode or grid 23A and a lower ion extraction grid 23B; 24, a microwave introducing window made of fused quartz; 25, a rectangular waveguide; 26, a magnetic coil; 27, 28 and 29, insulators; 30 and 31, magnetic shield members with a high permeability; 32, a gas inlet; 33, a microwave reflector; 34, a microwave coupling window; 35, an insulating spacer; and 36, a plasma transport chamber.
When the ion source shown in FIG. 2 is operated, first, a gas is introduced through the gas inlet 32 to the plasma formation chamber 21, while the microwave is introduced into the plasma formation chamber 21 through the rectangular waveguide 25, the microwave introducing window 23 and the microwave coupling window 34. At the same time, the magnet coil 27 produces a magnetic field which satisfies a condition of electron cyclotron resonance at least at one portion in the plasma formation chamber 21, so that plasma is produced in the plasma formation chamber 21. In order that the microwave power is efficiently absorbed by the plasma, the plasma formation chamber 21 is in the form of a cavity resonator in which the microwave reflector 33 is disposed, and the size and shape of the microwave reflector 33 is so selected that the microwave is reflected back by the microwave reflector 33 but the plasma can freely pass through the reflector 33 toward the plasma transport chamber.
In order to extract the shower-like ion beam 11 from the plasma formation chamber 21 and the plasma transport chamber 36 toward the specimen chamber 22, a positive potential is applied to the plasma formation chamber 21 and the lower ion extraction grid 23B of the ion extraction grid assembly 23 is grounded, while the upper ion extraction grid 23A is electrically floated. The insulating spacer 35 is provided so that the upper ion extraction grid 23A is electrically floated, and the insulators 27 and 28 are provided in order to apply a potential to the plasma formation chamber 21. The upper ion extraction electrode 23A is floated, so that a negative potential is induced in a self-regulation manner depending upon the electron energy in the plasma. As a result, the number of incident electrons is considerably reduced, so that the upper and lower ion extraction grids 23A and 23B are prevented from being heated and at the same time an abnormal discharge is suppressed. The magnetic shield member 31 with a high permeability covers the top and the outer periphery of the magnet coil 27 so that a diverging magnetic field which is gradually weakened toward the lower ion extraction grid 23B is produced within the plasma formation chamber 21, whereby an ion extraction efficiency can be improved.
According to Child's law, the relationship between an ion current density (J) of the ion current extracted from the plasma formation chamber 21 to the specimen chamber 22 and a voltage (V) applied between the plasma formation chamber 21 and the ion extraction grid assembly 23 may be approximated as follows: EQU J.varies.V.sup.3/2 .multidot.l.sup.-2
,where l is a distance between the ion extraction grids 6 and 7 or 23A and 23B. In the case of the ion shower apparatus of the type as shown in FIG. 1 or 2, the diameter of the ion extraction grids 6 and 7 or 23A and 23B is equal to or greater than that of the shower-like ion beam. The diameter is, in general, from 4 to 10 inches. In order that a uniform distance or gap may be maintained between the two ion extraction grids having such a large diameter as described above, the distance between the two grids is maintained to be 1 mm through 3 mm because thermal deformations must be taken into consideration. Therefore, it follows that in order to obtain the current density J which can attain a practical etching rate, a potential applied between the two ion extraction grids must be increased.
For instance, in case that the ion shower apparatus as shown in FIG. 2 is used for etching, when carbon fluoride gas (such as C.sub.2 F.sub.6, C.sub.4 F.sub.8) is introduced through the inlet 32 into the ion shower apparatus so as to etch an SiO.sub.2 film and when the voltage applied between the plasma formation chamber 21 and the grid 23B is 1000 V, a high etching rate of SiO.sub.2 film is obtained and a relatively high ratio of the etching rate of SiO.sub.2 film to an etching rate of Si is obtained. But, if the voltage applied between the plasma formation chamber 21 and the grid 23B is less than 500 V, the etching rate of a SiO.sub.2 film and the ratio of the etching rate of a SiO.sub.2 film to the etching rate of other material are low, which are not satisfactory in practice.
If the voltage applied between the two grids is increased for extracting ions for etching, a satisfactory etching rate and a satisfactory etching rate ratio can be obtained, but abnormal discharge tends to occur between the grids. As a result, ions extracted by a high potential damages crystal structures on the surface being etched so that the functions of the semiconductor devices are degraded. Thus, the ion shower apparatus with a two grid system has a disadvantage in that its application range is limited. There are also other disadvantages in that a pertinent ion current cannot be extracted in case of ion beam sputtering by introducing an Ar gas to the ion shower apparatus and in that it is difficult to determine a low voltage which is preferable to an ion beam deposition process. In addition, there is a disadvantage in that there is involved a difficult work of disposing and securing two grids in such a way that their apertures are correctly aligned or registered with each other. Moreover, it is difficult to obtain a uniform ion shower with a large diameter.
FIG. 3 schematically shows an ion source of an ion shower apparatus of the type in which only one ion extraction grid is used. That is, the upper grid of the ion shower apparatus as shown in FIG. 2 is eliminated. Such an ion shower apparatus is disclosed in detail in "Low Energy Ion Beam Etching", by J. M. E. Harper et al.; J. Electrochem. Soc., Vol. 128, No. 5, pp. 1077-1083 (1981). In FIG. 3, the same reference numerals as in FIG. 1 are used to designate the corresponding parts of the ion shower apparatus as shown in FIG. 1 are used to designate similar parts. It is seen in FIG. 3 that the upper ion extraction electrode 6 is eliminated. Reference numeral 14 designates a plasma sheath and 15, a sheath surface. In this apparatus, a plasma sheath is produced in a self-regulation manner and an ion beam is produced by the electric field in the plasma sheath. The sheath surface 15 is formed at the position at which the upper ion extraction grid 6 as shown in FIG. 1 is disposed and the thickness of the plasma sheath 14 corresponds to the distance l between the upper and lower ion extraction grids 6 and 7 as shown in FIG. 1. That is, the plasma sheath 14 acts as an imaginary upper ion extraction grid. The thickness of the plasma sheath 14 is dependent on the voltage applied to the ion extraction grid 7. For instance, when the applied voltage is 100 V, the thickness of the plasma sheath 14 is in the order of 0.1-0.2 mm. Therefore, as Child's law indicates, there exists an advantage that a high current density can be obtained even at a low potential.
However, in the ion shower apparatus shown in FIG. 3, when a potential is applied to the ion extraction grid 7, the electric field thus produced is extended into the whole plasma formation chamber 1 so that the discharge electric field established between the thermionic cathode 3 and the anode 4 for producing the plasma is considerably disturbed. As a result, abnormal discharges such as spark discharge tends to occur very frequently, and accordingly plasma cannot be sustained in a stable manner. Furthermore, according to Paschen's law, a spark-over occurs between the ion extraction grid 7 and particular a portion of the wall of the plasma formation chamber 1, so that a stable ion beam cannot be produced when a high potential higher than 100-200 V is applied to the ion extraction grid 7. Therefore, as compared with the ion shower device with a two-grid system as shown in FIG. 1, a practical etching rate, a good etching rate ratio of the etching rate to an etching rate of other material, and an ion current suitable for ion beam sputtering cannot be attained, even though the current density can be increased at a potential less than 100-200 V.
In addition, in order to produce a shower-like ion beam with a high degree of directivity, the diameter of the apertures of the ion extraction grid 7 is made substantially equal to the thickness (0.1-0.2 mm) of the plasma sheath. Consequently, the ion extraction grid 7 must be machined or fabricated with a high degree of accuracy, so that it is difficult to use the ion extraction grid 7 in practice. Furthermore, the size of the rim forming the ion extraction grid 7 must be substantially equal in dimension to or less than the fine apertures thereof, so that the ion extraction grid 7 is easily subjected to thermal damage. Consequently, there is a disadvantage in that a high current cannot be extracted.